Several apparatuses are known in the technical literature for the supervision of telephone circuits, line connections. The correct operation of the circuits is generally characterized by the average holding time of the circuits.
Accordingly the statistical monitoring of the holding time of the circuits is performed by the apparatus described in the U.S. Pat. No. 3,916,123. The drawback of this solution is that-- owing to the factors influencing the holding time--faulty circuits can be indicated only in case of extreme holding time maintained over a long period. The drawback is eliminated by the Swiss Pat. No. 615,790. According to this solution in addition to the nomal holdings (line seizures or usages), those exceeding a certain time are counted separately. The holdings exceeding a given time limit are regarded as effective holdings. The ratio of the holdings and effective holdings compared with the holding ratio of a group ensures the indication of the faulty circuits. However the solution did not eliminate the extremely extensive range of the times necessary for establishing the connection, thus selection of the threshold time is difficult, and the frequently occuring disconnection based on the timing, i.e. the connection exceeding the threshold is also regarded as effective.
An apparatus suitable for supervision of the sub-exchanges is known from the Swiss Pat. No. 614 580. This apparatus is connected to the wire characteristic to holding of the supervised circuit and to the wire relaying the pulses related to the tariff. The ratio of the holdings and the tariff indicates whether the examined circuit is faulty or not.
The shortcoming of the solution is that the range of the tariff pulses for each holding (line seizure or usage) is very extensive in the public network, consequently the circuit can be regarded as faulty only in case of considerable deviation of the ratio.